Abstract
Unlike most clinical academics, I have the opportunity of seeing a reasonable number of first opinion cases, as well as feline referrals. This gives me the opportunity of having regular clients and following their cats over the course of their natural lifespan — watching them grow from kittens into young adults, through middle age and subsequently into their geriatric years. Like cats everywhere, my patients commonly develop conditions related to their age, such as periodontal disease, degenerative joint disease, renal insufficiency, hyperthyroidism and cancers. Being in a University Veterinary Centre, I also have the opportunity of seeing and treating a very large number of feline hyperthyroid cases diagnosed by colleagues in practice, and subsequently referred for I-131 therapy.
Seeing an average of two cats with hyperthyroidism per week, one gets very adept at palpating the ventral cervical region for masses, and certainly there is a substantial variation in the number, size, consistency and location of ‘thyroid nodules’ in cats with symptomatic hyperthyroidism. Some cats have what feels like a single thyroid nodule (perhaps a thyroid adenoma, and typically more on one side). Others have two nodules, while still others have a series of small palpable goitrous lesions. Although my hospital does not have in-house scintigraphy, cursory ultrasound examinations of several cats with hyperthyroidism have indicated that some of the variation in the size of thyroid lesions is related to the number and size of the cysts that they contain. Indeed, a recent case was so large that 20 ml of cyst fluid was drained using a butterfly needle prior to radioiodine therapy! (Figures 1 and 2) Oftentimes scintigraphy or ultrasound will demonstrate that many more thyroid nodules are present than are palpable.
A hyperthyroid cat with an enormous cystic thyroid nodule.
Draining fluid from the cystic thyroid mass.
Over the years, it has been my observation that, at least on the east coast of Australia, a substantial number of ‘normal’ cats older than 10-years have palpable thyroid nodules. Typically, these are located in the ‘normal’ thyroid position just below the larynx, rather than the more ventral locations encountered in the majority of clinically hyperthyroid cats. Determination of plasma thyroxine concentrations in these asymptomatic cats typically reveals physiological concentrations of T4, as might be expected from their clinical euthyroid status. It has been my experience that if you follow these cats over time, their thyroid nodules tend either to remain ‘prominent’ or become progressively larger. Some of these cats eventually develop clinical signs referable to hyperthyroidism, in association with elevated plasma thyroxine concentrations and require treatment, ideally using radioiodine. My impression has been that this transition from an incidental nodule to symptomatic hyperthyroidism takes a couple of years, or more, to occur. Many cats with thyroid nodules, however, succumb to other diseases such as renal insufficiency or neoplasia before they develop symptomatic hyperthyroidism. Indeed for inexperienced clinicians, the presence of palpable thyroid nodules is often problematic in the investigation of the sick elderly cat, as it is tempting to blame the cat's weight loss on hyperthyroidism, when the real problem is undetected cancer or some other problem. Note, however, that all the aforementioned comments are anecdotal and unsupported by objective data!
Such observations, which no doubt have been made also by many readers of this journal, beg the question — what is the best way to handle the older cat in which a thyroid (or parathyroid) nodule is detected in the absence of signs of hyperthyroidism? In two papers in this edition of JFMS, Gary Norsworthy and collaborators provide conclusive evidence that a substantial proportion of elderly cats with thyroid lesions go on to develop clinical hyperthyroidism some time down-the-track. These observations confirm what many clinicians have long suspected, but it took the effort of a busy and able feline practitioner to follow-up enough cases to show unequivocally the progression from incidental goitre to symptomatic hyperthyroidism. A strength of this work was the tracking of individual patients over a sufficiently long time frame and the procurement of thyroid tissue via thyroidectomy to make a definitive diagnosis of the underlying disease process. (This is not possible in centres that routinely use radioiodine for therapy.) Interestingly, the occasional cat was found to have a non-functional parathyroid lesion rather than a thyroid lesion.
The results of Norsworthy's group will hopefully open up a new vista for feline endocrinologists in institutional practice, who will no doubt be interested in more precisely charting the transition from non-functional to functional goitre using scintigraphy, high-resolution ultrasonography and dynamic endocrine testing. Hopefully such studies will provide new information that will answer the substantial unresolved question — When should the older cat with an asymptomatic thyroid nodule be treated? Or put another way, how soon is it acceptable to surgically remove a thyroid mass, or administer a low dose of radioiodine, in a cat with an enlarging thyroid lesion. Must we wait for cats to develop overt signs of hyperthyroidism such as weight loss, a loud fast heart, hypertension and the like? Can we instead make a pre-emptive strike, administer a small dose of I-131 and nip the hyperplastic thyroid tissue in the bud?
Currently, my advice to clients owning asymptomatic cats with thyroid lesions is to monitor their cats for signs of ‘early’ hyperthyroidism and periodically determine the T4 level, perhaps annually or more often. Is this the right thing to do? Should cats have to develop signs of hyperthyroidism, and possibly risk irreparable damage to their myocardium, for example, before having their thyrotoxicosis treated? It seems to me that our feline patients are disadvantaged compared to their canine counterparts with Cushing's disease — which are now being diagnosed and treated much earlier in the disease process, well before they develop hair loss, a pot belly, diabetes and secondary infections. No doubt the two papers by Norsworthy and colleagues will stimulate debate, ideas for further research and ultimately provide data on which to make a more informed decision about how and when to treat this common and important endocrinopathy of cats.